Solid oxide fuel cells (SOFCs) are well known in the fuel cell art. SOFCs have significant advantages over some other forms of fuel cells, such as proton exchange membrane (PEM) fuel cells, in that SOFCs are not subject to CO poisoning of the anode and therefore can burn reformate directly from a catalytic hydrocarbon reformer, which produces CO and H2. Further, some simple fuels such as natural gas (methane) and short-chain alcohols (methanol, ethanol, etc.) can be partly or fully reformed internally in the SOFC anode, conferring large advantages in efficiency and simplification of an overall fuel cell system.
It is known in the fuel cell art to employ ammonia (NH3) as another cost-effective alternative fuel, which is well-matched to internal reforming. Ammonia may be fed directly into an SOFC anode and cracked to nitrogen and hydrogen in an endothermic reaction without requiring a separate catalytic fuel reformer as is needed for fueling of an SOFC with conventional, petroleum-based fuels.
A very great advantage of ammonia compared to carbon-based fuels is that coking of the anode is not possible, because there is no carbon in the fuel. This allows a wider window of operating temperature in the anode, which can make possible operation of an SOFC at lower operating temperatures, thereby permitting the use of lower cost materials for forming the SOFC stack.
Another advantage is that manufactured ammonia is essentially sulfur-free, which avoids the well-known deterioration of an anode that can occur with even trace quantities of sulfur, as are typically present in naturally-occurring hydrocarbon fuels such as gasoline. This feature also avoids the necessity of incorporating into an SOFC system sulfur-trapping means and/or expensive sulfur-tolerant materials of manufacture.
Another advantage of fueling by ammonia is that it is a zero CO2-emissions fuel. Although commercial synthesis of ammonia typically involves the use of hydrocarbon fuels, such synthesis may be performed at the hydrocarbon wellhead with the resulting CO2 sequestered underground. Alternatively, ammonia may be synthesized using nuclear energy which, of course, does not generate CO2.
A solid tablet invented by a Amminex A/S comprising ammonia absorbed efficiently in compact salt units is safe to handle and consist of inexpensive and abundant raw materials. This makes it different from most other hydrogen storage technologies in that it requires no special safety precautions. Ammonia has a high hydrogen content and when stored safely in a solid form enables the material to contain large amounts of hydrogen per unit volume, making it an ideal “hydrogen carrier” fuel for vehicular and stationary power systems.
It is known to combine an SOFC stack, operated with hydrocarbon reformate as a fuel source, with an internal combustion engine (ICE) in a two-stage energy conversion system wherein the anode tailgas of the SOFC, containing large amounts of hydrogen and CO, is used as a hot fuel for the downstream ICE.
What is needed in the art is an analogous system fueled by ammonia as a source for hydrogen fuel.
It is a principal object of the present invention to provide an efficient, zero-CO2-emissions, system for generating electrical, and optionally mechanical and thermal, energy.